On the history of graph theory. Origin
... authors believe that the first letter on graph theory by Karl Leonhard Gottlieb Ehler was sent to Leonhard Euler on March 9 (20), 1736 with the question of “the construction of the seven Konigsberg bridges“.
Matsievsky S. V., Kvitko G. V.
graph, genealogy, tree, constellation, hypergraph, Porphyry, Königsberg bridges problem, Euler